JPH04289718A - Digital protective relay - Google Patents

Abstract

PURPOSE:To dispense with an expensive analog filter which has been regarded as necessary in a conventional digital protective relay. CONSTITUTION:By setting the sampling frequency fs from 12fo till then (fo is basic frequency of AC signal) to 36fo, the level of the ZH' harmoinics whose frequency is above 18fo being falf of the sampling frequency fs is supressed to the one which can be neglected in the operation of a CPU 5, whereby the removal of the ZH' harmonics can be dispensed with, while ZL' harmonics not more than half the sampling frequency fs is removed with the digital filter function of the CPU 5.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital protective relay.

0002

[Prior art] Conventional digital protective relays include:
As shown in FIG. 2, an amplifier circuit 1, an analog filter 2, a sample/hold circuit 3, an A/D converter 4, and a CP
There is one made up of U5.

[0003] For convenience of explanation below, among the harmonics superimposed on the AC signal, 1/2 of the sampling frequency fs
(It is called the Nyquist frequency.) The harmonics below are called ZL harmonics, and the harmonics above 1/2 are called ZH harmonics.
FIG. 3 shows the harmonic spectrum of the AC signal. fo in FIG. 3 is the fundamental frequency of the AC signal.

In the conventional protective relay shown in FIG. 2, the sampling frequency fs given to the sample-and-hold circuit 3 is generally 12fo, and the Nyquist frequency is half of that, 6fo. Therefore, in the conventional digital protective relay, the ZL harmonic is less than 6fo in FIG. 3, and the ZH harmonic is more than 6fo.

[0005] In determining relay elements such as overvoltage and overcurrent in the protective relay, for example, AC signals such as voltage and current in the power line are taken in, and
The CPU 5 executes various calculation processes necessary for the above judgment, such as amplitude calculation, phase calculation, frequency calculation, etc. of the input AC signal, but since the input AC signal itself is in analog form, it is not possible to perform the calculation processing of the CPU 5 as it is. Not suitable.

[0006] Therefore, this AC signal is converted into a digital form that can be processed by the CPU. Then, the analog filter 2 removes the ZH harmonics in FIG. 3, and the removed AC signal is sampled and
While holding the data, it is converted into digital data by the A/D converter 4, and the CPU 5 uses this A/D conversion output as sampling data.
It is designed to output to .

[0007] The CPU 5 executes the various calculations described above from this sampling data, and during the execution, the ZL harmonics among the harmonics are removed using a digital filter function.

Here, since the sampling frequency fs is set to 12fo as mentioned above, the analog filter 2 removes the ZH harmonics of 6fo or more.
The CPU 5 will remove ZL harmonics below 6fo.

In this way, the harmonics are classified into ZH and ZL, and ZH is removed by the analog filter 2, and ZL is removed by the CPU 5.As is well known, the CPU 5 removes the ZH harmonics. This is because it could not be removed due to the principle of reproducing AC signals according to the sampling theorem.

0010

By the way, in order for the CPU 5 to accurately demodulate and reproduce the AC signal according to the sampling theorem for the above-mentioned various calculations, the sampling frequency fs ( Nyquist frequency 6 which is 1/2 of = 12fo)
For fo, it is necessary to sharply attenuate the AC signal by, for example, -40 dB or more and at a rate of, for example, -24 dB/oct.

However, an analog filter 2 that can limit the filter band and remove ZH harmonics with such a sharp attenuation characteristic inevitably requires an increase in the number of components and complexity. The problem was that it was expensive.

Therefore, in the present invention, the sampling frequency is set high so that harmonics at the Nyquist frequency are negligible in terms of the level of calculations performed by the CPU, and harmonics below the Nyquist frequency can be removed by the CPU. Since higher harmonics can be ignored in terms of level, analog filters are not used to remove the harmonics, thereby providing a digital protective relay that does not require expensive analog filters. The purpose is to

[0013]

[Means for Solving the Problems] In order to achieve such an object, the digital protective relay of the present invention has the following features:
A sample and hold circuit that samples and holds an AC signal at a predetermined sampling frequency, an A/D converter that converts the output of the sample and hold circuit into A/D, and a sampling theorem in response to the output from the A/D converter. and a CPU that demodulates and regenerates the AC signal and performs calculations for determining relay elements based on the regenerated AC signal, and the CPU is configured to perform calculations for determining relay elements based on the regenerated AC signals, and the CPU is configured to perform calculations for determining relay elements based on the regenerated AC signals. Assuming that the frequency of the harmonic when it reaches a level that does not affect the above is 1/2 of the sampling frequency, the sampling frequency output is output to the sample and hold circuit, and in the calculation, the A/D conversion output is It is characterized by having a digital filter function that removes harmonics of 1/2 or less of the included sampling frequency.

[0014]

[Operation] The CPU outputs the frequency of the harmonics superimposed on the AC signal to the sample-and-hold circuit as a sampling frequency when the harmonics reach a level that does not practically affect the calculation. Remove harmonics of 1/2 or less.

[0015]

Embodiments Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram of a digital protective relay according to an embodiment of the present invention. As shown in the figure, the protective relay according to the embodiment does not have the analog filter of the conventional example, and the output of the amplifier circuit 1 is sampled. The signal is output to the hold circuit 3. The sampling frequency fs given from the CPU 5 to the sample-and-hold circuit 3 is 36 times the fundamental frequency fo of the AC signal, that is, 36fo, and the CPU 5 is equal to or lower than the Nyquist frequency 18fo, which is 1/2 of the sampling frequency fs. It is characterized by having a digital filter function that removes the ZL' harmonic of. FIG. 3 shows the ZL' harmonic and the ZH' harmonic above the Nyquist frequency of 18fo.

[0016] At the Nyquist frequency 18fo,
The harmonics superimposed on the AC signal are calculated by the CPU 5 for determining the relay element (the calculation referred to in the prior art section above).
This has reached a level where it has no practical effect on the frequency, and therefore an analog filter for removing ZH' harmonics having a Nyquist frequency of 18fo or higher is not necessary.

The sampling frequency of the embodiment was three times that of the conventional example (36 times the fundamental frequency fo), but it was twice that of the conventional example (24 times the fundamental frequency fo). Of course, it may be 4 times (48 times the fundamental frequency fo), 5 times (60 times the fundamental frequency fo), or even more.

[0018]

[Effect of the invention] As is clear from the above explanation,
In the present invention, the sampling frequency for the sample-and-hold circuit is increased, and the harmonics superimposed on the alternating current signal at a frequency of 1/2 or more are made to a level that does not practically affect the calculations of the CPU. This eliminates the need for expensive analog filters, which had been a problem in the past.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a digital protective relay according to an embodiment of the present invention.

FIG. 2 is a block diagram of a conventional digital protective relay.

FIG. 3 is a diagram showing a spectrum of harmonics included in an AC signal.

FIG. 4 is a diagram showing filter characteristics of a conventional analog filter.

[Explanation of symbols]

1 Amplifier circuit 3 Sample/hold circuit 4 A/D converter 5 CPU

Claims (1)

[Claims] [Claim 1] A sample and hold circuit (3) that samples and holds an AC signal at a predetermined sampling frequency.
and an A/D converter (4) that A/D converts the output of the sample/hold circuit (3), and demodulates and reproduces the AC signal according to the sampling theorem in response to the output from the A/D converter (4). It also includes a CPU (5) that performs calculations for determining relay elements based on the reproduced AC signal,
The CPU (5) assumes that the frequency of the harmonics superimposed on the alternating current signal reaches a level that does not practically affect the calculation is 1/2 of the sampling frequency, and sets the sampling frequency output to the above. In addition to outputting to the sample/hold circuit (3), the above A/
A digital protective relay characterized by having a digital filter function that removes harmonics of 1/2 or less of the sampling frequency included in the D-converted output.